Chrisy Bossie built a $100,000-a-year gemstone e-commerce business by sharing information about her products on her company’s Facebook page several times a week.

“Steals in the Shop! I have a TON of new 36-inch-long necklaces, most priced at $15, available in amethyst, lapis, watermelon tourmaline, turquoise.... Shop them all here,” she wrote in a recent marketing post on a Facebook page for Earthegy, the business she runs from her home in rural Kent Store, Va. She also included photos and links to the products, hoping the...